Actuatable passenger restraint systems for vehicles are well known in the art. One particular type of actuatable passenger restraint system includes an inflatable air bag mounted within the passenger compartment of the vehicle. The air bag has an associated, electrically actuatable ignitor, referred to as a squib. Such systems further include an inertia sensing device for measuring the deceleration of the vehicle. When the inertia sensing device is subjected to a forcing function greater than a predetermined value, the inertia sensing device closes an electrical switch causing an electric current of sufficient magnitude and duration to be passed through the squib to ignite the squib. The squib, when ignited, ignites a combustible gas generating composition or pierces a container of pressurized gas, which results in inflation of the air bag.
Many known inertia sensing devices used in actuatable passenger restraint systems are mechanical in nature. Such devices are typically mounted to the vehicle frame and include a pair of mechanically actuatable switch contacts and a resiliently biased weight. The weight is arranged such that when the vehicle decelerates, the weight physically moves relative to its mounting. The greater the amount and duration of the deceleration, the further the weight moves against the bias force. The switch contacts are mounted relative to the biased weight such that, when the weight moves a predetermined distance, the weight moves over or against the switch contacts causing them to close. The switch contacts, when closed, connect a squib to a source of electrical energy sufficient to ignite the squib.
Still other known actuatable passenger restraint systems for vehicles include an electrical transducer or accelerometer for sensing vehicle deceleration. Such systems include a monitoring or evaluation circuit connected to the output of the transducer. The transducer provides an electrical signal having a value proportional to the vehicle's deceleration. The monitoring circuit processes the transducer output signal. One typical processing technique is to integrate the transducer output signal sometimes clipped at and/or offset by predetermined values directly by use of an analog integrator. If the output of the integrator exceeds a predetermined value, thereby indicating crash velocity greater than a certain amount, an electrical switch is actuated to connect electrical energy to the squib.
One example of a passenger restraint system using an electrical accelerometer is disclosed in U.S. Pat. No. 3,870,894 to Brede, et al. ("the '894 patent"). The '894 patent discloses a system which includes an accelerometer, an evaluation circuit connected to the accelerometer, and an ignition circuit or squib connected to an output of the evaluation circuit. The accelerometer includes a piezoelectric transducer that provides an electrical output signal having a value proportional to the vehicle deceleration. The evaluation circuit includes an integrator electrically coupled to the output of the accelerometer through an amplifier. The output of the integrator is an electrical signal having a value proportional to the integral of the deceleration signal. A trigger circuit is connected to the output of the integrator. When the output of the integrator reaches a predetermined value, the trigger circuit actuates a time delay circuit. The time delay circuit begins to time out a predetermined time period. After the time period is timed out, the air bag ignition circuit is energized.
It has been discovered that it is not desirable to inflate a vehicle air bag under all types of crash conditions to which the vehicle is subjected. It is not desirable, for example, to inflate the air bag during certain types of low speed crashes. Such a crash is referred to as a non-deployment crash. A non-deployment crash is one in which it is not desirable to deploy the vehicle air bag. Similarly, a deployment crash condition is one in which it is desirable to deploy the vehicle air bag. The determination as to what crash conditions fall within the definition of a non-deployment crash is dependent upon various factors related to the type of vehicle. If, for example, a small or medium size vehicle were to hit a brick wall at 30 miles per hour, such a crash condition would be a deployment crash. On the other hand, if a large vehicle traveling eight miles per hour hits a parked vehicle, such a crash would be considered a non-deployment crash condition. During such a non-deployment crash condition, a typical accelerometer would provide an output signal indicating a large deceleration is occurring. In an actuatable passenger restraint system using an integrator connected to an accelerometer wherein the integrator is the only evaluation device, the air bag would be inflated as soon as a sufficient speed differential occurred that would result in the integrator output exceeding a predetermined limit. If the threshold level for triggering inflation were increased so that the system did not inflate during this non-deployment event, the resulting threshold may be too high for some types of deployment crashes (for example pole and angled crashes) and the system would actuate too late.